U.S. Pat. No. 5,458,642 (Beer) discloses an intervertebral disc prosthesis comprising an upper and a lower kidneys-shaped plate, these plates being each peripherally connected (about a central core) to each other by a plurality of helical springs. The helical springs are configured in equidistant manner to each other and allow displacing said plates in three dimensions within given limits. However this known intervertebral disc prosthesis incurs the drawback that the stiffness of the implant is identical in every radial direction—at the same distance from the implant center, in other words, the implant is symmetrically stiff.